The present invention relates to an improved support tray for use, for example, in the cleaning of wall-mounted air conditioner units, particularly for use in the cleaning of the well-known packaged terminal air conditioners (hereinafter termed “PTAC”) that are commonly installed in hospitality settings. It also relates to the use of a dust containment bag during the cleaning of such air conditioner units, while the unit is sitting on the support tray of this invention.